Dear Yvan,
I would like to clarify how steady and unsteady RANS are selected in Code_Saturne. Am I correct that choosing Constant or Adaptive time step corresponds to unsteady simulations, while Steady (local time step) corresponds to a steady simulation (But why does the Steady option still require input of adaptive time-step parameters, is it the “pseudo-time”)?
In my case, I used the Adaptive time step option for a wall-bounded square cylinder. The flow should be unsteady, but the results appear nearly steady: the time history shows almost no variation, and the snapshots do not reveal vortex shedding. Do you have any suggestions on how to address this? Is there a specific option I need to enable for URANS?
For reference, I am using Code_Saturne 8.0 with an ABL inlet profile and wall functions at the wall. My setup.xml file is attached.
Best regards,
Ximeng
Question on Steady vs Unsteady RANS in Code_Saturne
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Question on Steady vs Unsteady RANS in Code_Saturne
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- setup_domain2Larger_firststage_RANSkwsst_wallfun_LogProfile_coarsemeshz1e-3xy1.5e-3.xml
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Yvan Fournier
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- Joined: Mon Feb 20, 2012 3:25 pm
Re: Question on Steady vs Unsteady RANS in Code_Saturne
Hello,
The "steady" option is actually (as mentioned) "pseudo-steady", because it is not based on a steady algorithm, but on an unsteady algorithm with a local time step which allows faster convergence to steady state.
In most cases, the fact that a RANS computation leads to stable or fluctuating results depend on a combination of that case's physical behavior, time step/CFL aspects, numerical options, and mesh complexity and quality.
When you do observe fluctuations, they should be compared to the turbulence model's local time scale, to determine whether this may make physical sense (intepret with caution), or have a period which is smaller than that ime scale, in which case they can be considered as numerical artifacts/parasites.
If a case is very stable, this might simply be because the actual physical behavior is stable,
Regards,
Yvan
The "steady" option is actually (as mentioned) "pseudo-steady", because it is not based on a steady algorithm, but on an unsteady algorithm with a local time step which allows faster convergence to steady state.
In most cases, the fact that a RANS computation leads to stable or fluctuating results depend on a combination of that case's physical behavior, time step/CFL aspects, numerical options, and mesh complexity and quality.
When you do observe fluctuations, they should be compared to the turbulence model's local time scale, to determine whether this may make physical sense (intepret with caution), or have a period which is smaller than that ime scale, in which case they can be considered as numerical artifacts/parasites.
If a case is very stable, this might simply be because the actual physical behavior is stable,
Regards,
Yvan
Re: Question on Steady vs Unsteady RANS in Code_Saturne
Sorry, as I can see, he asks how to switch to transient case (with inner iterations on each timestep) using time stepping options in GUI. Something like Constant time step + Inner iterations.
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Yvan Fournier
- Posts: 4258
- Joined: Mon Feb 20, 2012 3:25 pm
Re: Question on Steady vs Unsteady RANS in Code_Saturne
Hello,
Inner iterations (nterup) can be used in the velocity/pressure coupling algorithm mostly for FSI computations, but I do not think they ate required for URANS. The same, sweeps for each solver are used to better handle mesh non orthogonalty (where some reconstruction terms are handled explicitly by increments in sub-iteratiobs), and for transient flows, this is recommended over than using relaxation, as it avoids degrading the time scheme.
In code_saturne, I am not aware of sub-iterations being needed for unsteady RANS. There is actually nothing specific separating URANS from RANS in code_saturne, at least from what I understand from conversations with turbulence specialists. I'll check with them in case there is a subtle difference I forgot.
Regards,
Yvan
Inner iterations (nterup) can be used in the velocity/pressure coupling algorithm mostly for FSI computations, but I do not think they ate required for URANS. The same, sweeps for each solver are used to better handle mesh non orthogonalty (where some reconstruction terms are handled explicitly by increments in sub-iteratiobs), and for transient flows, this is recommended over than using relaxation, as it avoids degrading the time scheme.
In code_saturne, I am not aware of sub-iterations being needed for unsteady RANS. There is actually nothing specific separating URANS from RANS in code_saturne, at least from what I understand from conversations with turbulence specialists. I'll check with them in case there is a subtle difference I forgot.
Regards,
Yvan